For example, Patent Literature 1 discloses a driver solenoid spool valve configured to cause a linear solenoid to drive a spool. Downsizing of a solenoid spool valve is demanded to enhance a mountability. A solenoid spool valve is equipped to, for example, a hydraulic pressure control device of an automatic transmission device.
A valve body is equipped inside an automatic transmission device. The valve body has a passage circuit having multiple passages. The valve body is equipped with multiple solenoid spool valves in order to switch the passages and to control a hydraulic pressure. In recent years, downsizing of a solenoid spool valve is demanded with downsizing of an automatic transmission device and/or increasing in the number of stages of the automatic transmission device. In particular, a number of linear solenoids are equipped in a valve body, and therefore, downsizing of the linear solenoids is demanded. In consideration of this, various ideas are proposed to downsize a conventional linear solenoid. However, a limit exists in downsizing of a linear solenoid by enhancing an efficiency of a magnetism in the linear solenoid, and therefore, further downsizing of a linear solenoid is difficult.